The bound package

We represent the target language itself as an ideal monad supplied by the
user, and provide a Scope monad transformer for introducing bound variables
in user supplied terms. Users supply a Monad and Traversable instance, and
we traverse to find free variables, and use the Monad to perform substitution
that avoids bound variables.

The classes from Prelude.Extras are used to facilitate the automatic deriving
of Eq, Ord, 'Show, and Read in the presence of polymorphic recursion used
inside Scope.

The goal of this package is to make it as easy as possible to deal with name
binding without forcing an awkward monadic style on the user.

With generalized de Bruijn term you can lift whole trees instead of just
applying succ to individual variables, weakening the all variables bound
by a scope. and by giving binders more structure we can permit easy
simultaneous substitution.

However, the combinators they used required higher rank types. Here we
demonstrate that the higher rank gfold combinator they used isn't necessary
to build the monad and use a monad transformer to encapsulate the novel
recursion pattern in their generalized de Bruijn representation. It is named
Scope to match up with the terminology and usage pattern from Conor McBride
and James McKinna's "I am not a number: I am a free variable", available from
http://www.cs.st-andrews.ac.uk/~james/RESEARCH/notanum.pdf, but since the
set of variables is visible in the type, we can provide stronger type safety
guarantees.

Simple.hs provides an untyped lambda calculus with recursive let bindings.
and includes an evaluator for the untyped lambda calculus and a longer example
taken from Lennart Augustsson's λ-calculus cooked four ways available from
http://www.augustsson.net/Darcs/Lambda/top.pdf

2. Derived.hs shows how much of the API can be automated with DeriveTraversable
and adds combinators for building binders that support pattern matching.

3. Overkill.hs provides very strongly typed pattern matching many modern type
extensions, including polymorphic kinds to ensure type safety. In general,
the approach taken by Derived seems to deliver a better power to weight ratio.